We propose to develop technologies for making targeted gene inactivations in mice ten to fifty fold faster than is currently possible. Such "knockout" transgenic mice are critical for understanding gene function and disease processes at the organismal level. Instead of conventional approaches that rely on restriction enzymes to construct targeting vectors by recombinant DNA procedures, novel plasmids and methods for harnessing the recombinational machinery of bacteria to rapidly assemble targeting vectors will be developed. These vectors will be more precisely constructed and will be at least as efficient at homologous recombination than conventional targeting vectors. PROPOSED COMMERCIAL APPLICATION The development of efficient, large-scale gene targeting technologies will accelerate gene discovery and gene functional analysis providing for more rapid development of diagnostics and therapeutics for human genetic diseases. Valuable animal models for diseases will also be generated.